An earthquake that rattled the Tri-Valley Friday morning was an aftershock from one that shook the area Thursday, according to a U.S.
Geological Survey seismologist.
Friday's quake occurred at 11:48 a.m., according to the USGS, and had a depth of five miles. After initially estimating a preliminary magnitude of 3.8, the agency quickly downgraded it to 3.7, said USGS seismologist David Oppenheimer.
The quake was centered in the same area as the previous day's quake, which had a preliminary magnitude of 4.1, said Oppenheimer. The pair
of temblors occurred on two small, separate fault lines, both located about
six miles east-northeast of Milpitas.
Initially, seismologists thought the Thursday quake occurred on the Calaveras Fault, which runs through the region. However later in the day they determined it was actually centered on a small, nameless fault that lies perpendicular to the Calaveras, Oppenheimer said.
"Every earthquake has its own aftershock sequence," Oppenheimer said. He recalled a 4.4 magnitude earthquake in the Alamo area in 1990 that recorded 350 aftershocks over 42 days.
"If it continues on like this and has a robust aftershock sequence, it could evolve into a swarm," he said. This phenomenon consists of "lots of earthquakes of similar magnitude; it can go on for weeks."
Conversely, Friday's could be the last of it, he said.
Within seconds of the earthquake, social media outlets like Twitter were buzzing about the follow-up quake, and many people expressed fear that they might signal an even larger earthquake in the near future.
The probability of that is very small, Oppenheimer said. "About 95 percent of all earthquakes don't have foreshocks."
Utilities crews say the two recent earthquakes have not caused any detectible damage to the Calaveras Dam, a 75-year-old structure holding back a reservoir that straddles Santa Clara and Alameda counties.
A 4.1-magnitude earthquake Thursday and a 3.7-magnitude quake Friday were both centered along the Calaveras Fault near the Calaveras Reservoir, according to the U.S. Geological Survey.
The San Francisco Public Utilities Commission dispatched workers and found no signs of damage to the reservoir infrastructure, including Calaveras Dam, an earth-filled structure built in 1925.
Water in the reservoir is currently lowered by 60 percent of full capacity due to concerns about the seismic stability of the dam.
"The timing of this tremor highlights the importance of our ongoing Water System Improvement Program to repair, upgrade and seismically reinforce these facilities," SFPUC General Manager Ed Harrington said in a statement.
"This work includes replacing Calaveras Dam, upgrades to the nearby treatment plant and strengthening the pipelines that cross this vault in the valley," Harrington said.
The project to replace Calaveras Dam is in environmental review, according to the SFPUC.